1. Field of the Invention
The present invention relates to an image forming apparatus and a fixing temperature control method.
2. Description of the Related Art
Conventionally, the rolling transfer method has been used for the image forming apparatus such as the electro-photographic printer and the electro-photographic copying machine. The rolling transfer method is designed to transfer the toner image formed on an image carrier, such as the photosensitive drum or the like, onto a sheet-like recording material such as a sheet of paper or the like. In the rolling transfer method, a conductive and elastic transfer roller is pressed against the image carrier with a total pressure of about 5-20N thereby to form a transfer nipper section between the image carrier and the transfer roller. The transfer nipper section is designed to nip and transfer the recording material to have the toner image formed on the image carrier transferred onto the recording material by the effect of the transfer voltage (transfer bias) applied to the transfer roller.
Besides, the heat roller method, the film heating method or the like are in use as the method for fixing the toner image (unfixed image), which has been transferred onto the recording material previously, by the effect of the heat. The heat roller system comprises a heating roller (image fixing roller), whose temperature is maintained at a predetermined level, and a pressure roller, which is provided with an elastic layer and pressed against the heating roller to form a fixing nipper section. When the recording material to be transferred is introduced between the rollers constituting the fixing nipper section, the toner image can be fixed on the recording material owing to the heat of the heating roller.
The film heating method is characterized by a system comprising a heater, a film (hereinafter referred to as “a fixing film”), which slides against the heater, and a pressure applying member, which forms, through the sliding film, the fixing nipper section in combination with the heater (Refer, for example, to Japanese Patent Application Laid-open No.4-44075(1992)). The recording material carrying the toner image is introduced into the fixing nipper section, whereby the toner image is thermally fixed on the recording material owing to the heat from the heater. For the heater, one having a low heat capacity and a high heat conductivity, such as a ceramic heater, is used. For the fixing film, a thin film having a low heat capacity is used. In this way, the film heating method reduces a time for setting temperature which the toner image is fixed on the recording material and provides saving of energy.
In the case of the image fixing unit using the film heating method, when the recording material having a high hygroscopic property is introduced and heated at a high temperature, the moisture contained in the recording material evaporates. The evaporation of the moisture in a large quantity gives rise to a problem such as the occurrence of slipping between the recording material and the fixing film or between the recording material and the pressure member. Further, the pressure and the high temperature required for fixing the image can cause the increase in the degree of the curling of the recording material. Increase in the degree of curling results in the decrease in the quantity of the recording material loadable on the ejected sheet tray, thereby giving rise to a problem such as the falling of the overloaded recording material.
In this connection, there is known a method designed so that the set temperature of the image fixing unit can be lowered from the initial setting when the value of the current based on the voltage applied to the transfer roller for the fixing of the image is found to be higher than the predetermined value (Refer, for example, to the Japanese Patent Application Laid-open No. 2001-290316). In general, there is an inverse relationship between the level of the hygroscopic property and the level of the surface resistance of the recording material. The rise of the moisture level in the recording material while the transfer voltage is applied to the recording material results in the increase in the value of current flowing in the transfer roller, and so it is reasonable to lower the set temperature of the image fixing unit when the recording material having a high hygroscopic property is to be processed. In this way, the occurrence of the problem as is discussed above can be prevented by controlling the generation of the water vapor resulting from the rapid rise of the temperature.
On the other hand, however, as long as the set temperature of the image fixing unit is uniformly controlled according to the hygroscopic property of the recording material, it is not always possible to set the temperature to an optimum level depending on the property of the recording material on which the toner image is to be transferred.
For instance, some of the small-size recording materials such as the postcards and envelopes have larger thickness and larger heat capacity than those of the ordinary paper sheets. Fixing the image on a small-size printing material requires a larger quantity of heat than that required for the ordinary sheet of paper. However, in the cases of the conventional arts, there is the possibility that the fixing temperature is set to the level lower than the necessary level even for the small-size recording material shaving a relatively high hygroscopic properties, thereby ending with insufficient fixing stability.
Further, (in the cases of the conventional arts,) there is the possibility that the slippery condition occurs between the recording material and the fixing film or between the recording material and the pressure member owing to evaporation of the moisture contained in the recording material from the non-image area of the fixing nipper section, especially when the print rate of the toner image transferred onto the recording material is relatively high. Further, even in the case of the recording material having a relatively high hygroscopic property, when the print rate of the toner image on the recording material is relatively high, the relatively large quantity of the toner on the recording material and the resulting high surface resistance causes the value of the current flowing in the transfer roller to become relatively small. That is, in the case of the above-mentioned prior art, the resistance of the recording material is determined to be high, so that the set temperature of the image fixing unit remains at a high level. This causes the evaporation of the moisture contained in the recording material and the resulting slippery condition that causes insufficient transfer of the recording material and the problems such as poor reproduction of the image or the jamming of the paper sheets.